1. Field of the Invention
The present invention relates to a method for producing (meth)acrylic acid. More specifically, the present invention relates to a method for producing (meth)acrylic acid using a process of recovering (meth)acrylic acid as aqueous (meth)acrylic acid solution from a (meth)acrylic acid-containing gas mixture produced by the catalytic gas phase oxidation of at least one reactant selected from the group consisting of propane, propylene, isobutylene and (meth)acrolein.
2. Description of the Prior Art
Conventionally, (meth)acrylic acid is obtained by the partial oxidation of propane, propylene, isobutylene and/or (meth)acrolein with a heterogeneous oxidation catalyst in the presence of water vapor. In this oxidation method for producing (meth)acrylic acid, by-product impurities, such as water or unreacted propane, propylene, isobutylene and (meth)acrolein, acetic acid, formic acid, formaldehyde, acetaldehyde, maleic acid, propionic acid, furfural and the like, are generated. A (meth)acrylic acid-containing gas mixture containing such by-product impurities is generally collected as (meth)acrylic acid solution via contact with an absorption solvent, and the solvent is separated by distillation, etc. Then, low-boiling point and high-boiling point components are selectively separated.
Methods for recovering (meth)acrylic acid from (meth)acrylic acid-containing gas by an absorption solvent, which have been known till now, can be broadly divided into methods using an organic solvent (e.g. U.S. Pat. Nos. 3,932,500 and 6,498,272) and methods using water or aqueous solution as solvent (e.g. Japanese Patent Publication No. Sho51-25602 and Japanese Laid-Open Patent No. Hei9-157213). Such recovering methods according to the prior art are known as methods for recovering acrylic acid as solution from acrylic acid-containing gas with high selectivity. There is a difference in concentration of acrylic acid in a recovered solution depending on the method used particularly.
U.S. Pat. No. 3,932,500 discloses a process comprising absorbing acrylic acid from an acrylic acid-containing product gas with a high-boiling, hydrophobic organic solvent, recovering acrylic acid from the absorbed solution and recycling the solvent to an absorption column. In this process, the concentration of acrylic acid at the bottom the absorption column is as low as 6-15% by weight, the amount of water contained in the absorbed solution is about 5% by weight, and the concentration of acrylic acid in the off-gas from the absorption column is about 1%. This loss of acrylic acid (˜1%) at the top of the absorption column is connected directly with process economy and is burdensome considering that acrylic acid should be treated in subsequent processes without loss. Especially, in processes with larger-scale production, the loss of acrylic acid will not be cost-efficient. In order to increase the absorption of acrylic acid, the flow rate of a solvent for absorption needs to be increased. In this case, however, the concentration of acrylic acid in a solution obtained from the bottom of the absorption column will be reduced so that the flow rate of the solvent to be separated from acrylic acid solution in subsequent processes will be increased, resulting in inefficiency.
Japanese Patent Publication No. Sho 51-25602 discloses a process that comprises absorbing an acrylic acid-containing reaction product gas with water and recycling some of nitrogen, oxygen and water discharged from an absorption column to a reactor in order to adjust the gas concentration required for catalytic oxidation (see FIG. 4). This process has an advantage in that the circulating supply of water required in the reactor is possible since acrylic acid is absorbed with water in the absorption column. Also, the concentration of acrylic acid at the bottom of the absorption column is 40-80% by weight, and generally 60-70% by weight. Furthermore, the loss of acrylic acid vented from the absorption column is lower than that of the above-described absorption process using the organic solvent.
Other methods for recovering (meth)acrylic acid include methods in which quenching with an acrylic acid-containing solution is combined with absorption using water, aqueous solution or organic solvent (e.g. EP 9,545 and U.S. Pat. Nos. 4,554,054 and 6,498,272). For example, acrylic acid-containing gas obtained by catalytic gas phase oxidation at a high temperature of between 150° C. and 200° C. is quenched with acrylic acid-containing solution having a temperature of between 60° C. and 150° C. Then, uncondensed gas is discharged and recovered in a subsequent step via absorption using a solvent. EP 9,545 discloses a method of recovering acrylic acid carried out in a discrete and sequential mode or an integral mode, which comprises a step of quenching of acrylic acid-containing gas and a step of absorption using water. In this method, the concentration of acrylic acid in aqueous acrylic acid solution at the bottom of the recovering system is as low as about 60% by weight.